Activated carbon is a nongraphitic microcrystalline form of carbon which has been processed to produce a carbon with high porosity. Activated carbon is used widely in applications involving removal of impurities from liquids and gases. Activated carbon has particular utility in adsorbing and purifying fluid emissions from internal combustion engines.
Conventionally activated carbon is used in powdered or granular form. Powdered or granular activated carbon is inconvenient to use in processes where continuous flow of fluids are filtered and/or treated. To solve this problem, attempts have been made to use activated carbon in the form of, or in conjunction with a solid substrate.
For example, attempts have been made to manufacture monolithic bodies consisting essentially of activated carbon, or to extrude carbonaceous material into a shaped body, and then convert the shaped body to activated carbon. In such processes, a binder is typically added to activated carbon powder and the mixture is shaped by extrusion. One disadvantage of these bodies is that the binder blocks the pores of the activated carbon and therefore the adsorption capacity of the body is diminished. If the amount of binder is reduced to minimize blocking, the strength of the body is compromised. Furthermore, most substances useful as extrusion binders begin to deteriorate at temperatures above 150.degree. C. further diminishing their applicability. Also components of a workstream, and even water often react with binder.
More recently, activated carbon has been produced as a coating on a substrate, typically a honeycomb as described in EPO application publication no. 608,539 A1. According to this process, a carbon precursor liquid, commonly a thermosetting resin, is coated onto a substrate, and thereafter, the carbon precursor is cured, carbonized, after which the carbon can be activated. The resulting product is an interpenetrating network structure of substrate and carbon. The product is useful in many industrial and automotive applications.
In the above process, one very important requirement of the resin is that it should be of sufficiently low viscosity to impregnate the substrate structure. The resin viscosity has a strong effect on the amount of resin picked up by the substrate, which in turn determines the amount of carbon in the structure and hence its adsorption capacity. To make a consistent product it is necessary to maintain resin viscosity at a consistent level. If the resin has a long shelf life with a latent catalyst, then the viscosity can be maintained constant over a desired period of time. In case of some of the resins, maintaining viscosity at a constant level is not possible. Furan resins, for example, are very desirable thermoset resins for fabrication of carbon-ceramic honeycomb structures because of their high carbon content, high cross-linking density, and ease of cure. If these resins are thermally cured and carbonized without a catalyst, the carbon yields are low. The carbon yields can be increased by adding a catalyst and consolidating the resin before carbonization. The catalyst addition, however, results in continuous increase in viscosity which causes two problems. First the shelf life of the resin is very limited and second, the amount of resin pick up on the honeycomb continuously varies as a function of time thus making it impossible to make a consistent product.
The present invention provides a method which solves this problem.